rpmsg: glink: Add rx done command

Send RX data receive ack to remote and also inform that local intent
buffer is used and freed. This informs the remote to request for next
set of intent buffers before doing a send operation.

Acked-by: Arun Kumar Neelakantam <aneela@codeaurora.org>
Signed-off-by: Sricharan R <sricharan@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>